Attention has recently been focused on fuel cells as an energy supply source which has a high power generation efficiency and theoretically yields only water as a reaction product, while being excellent in environmental friendliness. Depending on species of electrolytes employed, such fuel cells are roughly classified into low-temperature operating fuel cells such as those of alkali, solid polymer, and phosphate types, and high-temperature operating fuel cells such as those of molten carbonate and solid oxide types. Among them, polymer electrolyte fuel cells (PEFCs) using a solid polymer as their electrolyte, which can attain a high density/high output in a compact structure while being operable in a simple system, have been widely studied not only as a stationary distributed power supply but also as a power supply for vehicles and the like, and have been greatly expected to come into practical use.
One of such PEFCs is a direct alcohol fuel cell which directly uses an alcohol as its fuel, in which a direct methanol fuel cell (DMFC) using methanol as its fuel has been known in particular. When methanol and water are supplied to an anode (fuel electrode) of the DMFC, methanol is oxidized by water, so as to generate a hydrogen ion. The hydrogen ion migrates through the electrolyte to a cathode (air electrode), thereby reducing oxygen fed to the cathode. According to these redox reactions, a current flows between both electrodes.
Thus, the direct alcohol fuel cell can directly use alcohol, which is a fuel, for power generation without modifying it into hydrogen and the like, and thus has a simple structure without necessitating a separate device for fuel modification. Therefore, the direct alcohol fuel cell can be made smaller and lighter very easily, and can favorably be used for a portable power supply and the like.
As a polymer electrolyte membrane for such a direct alcohol fuel cell, proton-conducting ion exchange membranes are usually employed, among which ion exchange membranes made of perfluorocarbon polymers having sulfonate groups are widely used in particular. On the other hand, each of the anode and cathode is constructed, for example, by two layers, i.e., a catalyst layer to become a reaction site for an electrode reaction and a diffusion layer for supplying a reactant to the catalyst layer, giving/receiving electrons, and so forth.
However, such a direct alcohol fuel cell has been known to cause the following problem, since alcohol is directly supplied to the anode. Namely, so-called “crossover” occurs, in which alcohol infiltrates the electrolyte membrane and reaches the cathode because of a high affinity of the solid polymer electrolyte membrane to alcohol and a concentration gradient. While platinum or the like which is highly active in oxygen reduction is employed as a catalyst in the cathode, alcohol having reached the cathode is immediately oxidized on platinum, so as to produce aldehydes, carbon monoxide, or carbon dioxide. Therefore, when the crossover occurs, the cathode attains a mixed potential of oxygen reduction and the oxidation of alcohol as mentioned above, and thus lowers the potential, thereby decreasing the cell voltage.
Thus, the crossover phenomenon has become a major cause of deterioration in performances of direct alcohol fuel cells. Therefore, various studies concerning the electrolyte membrane have been made in order to suppress the crossover. For example, Patent Document 1 discloses an electrolyte membrane containing a metal oxide, Patent Document 2 discloses an arrangement of a limiting permeable layer for restricting the permeation of a liquid fuel between a cathode and a solid electrolyte membrane, and Patent Document 3 discloses that an electrolyte membrane constituted by a first electrolyte layer and a second electrolyte layer which is less permeable to organic fuels than is the first electrolyte layer is arranged such that the first electrolyte layer is on the anode side. These methods suppress the crossover.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-331869    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-317742    Patent Document 3: Japanese Patent Application Laid-Open No. 2002-56857